I was wondering what the highest ratio can go and still use normal gas?
Thanks

 

I have heard you shouldnt go over 10.5:1 on 93 octane, but there will always be people disputing this. some people say 10.6 some say 10.7 and some say 10.0. I have just heard that 10.5:1 is the limit

 

It's like anything else in the world. It depends. It depends mostly on your heads. Aluminum heads can permit higher compression ratios without detonation. Fast-burn style combustion chambers will also permit higher compression. My old 461 fuelie heads are causing big problems with detonation/timing.

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This depends greatly on your combo. I would not go over 10.5:1 I would try and stay right around 10.0.

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gutted MAF,
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Flat top pistons will also accept a little more compression than a dome piston because of the better flame front across the piston top.

Theoretically with a properly built engine and todays electronic fuel management systems, you could easily get 12:1 on 93 pump gas. Aftermarket knock sensor timing computers will allow high compression because of how they control spark knock.

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87 IROC-Z Pro ET Bracket Race Car and knocking on the SuperPro ET class
383 stroker (carbed) with double hump cast iron heads and pump gas
LS6 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857
Best corrected ET: 11.163
Best MPH on a time slip: 117.87
Best corrected MPH: 126.10
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car
91 454SS daily driver
95 Homebuilt Harley

 

As a general rule of thumb, 9.5 is the limit for iron heads, 10.5 for aluminum. you can get a way with a few more if you run rich, retard timing, run a big cam etc, etc, etc. But, it's realy not worth it. Todays gas sucks so bad you never know what you're really getting, and knocking WILL destroy an engine eventually.

 

is there a faq on this yet? seems to be asked about once a week. it all depends on the application and parts used. i won't give numbers though, just thought this is asked quite often.

 

Another thing to add is the bore and stroke combination. A short strokin 327 will allow more compression than a comparable long strokin 400. Of course this is only part of the combination it takes to run high compression.


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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

 

Last month I built a 415 for my 86 Vette and I had the same concerns about compression.

I read all the mags I can get my hands on and have been building Chevy engines for more than 25 years, but there's always something new to learn.

When I did the math on the CR I really became concerned that it would be too high - 11.38:1, but decided to see just how far I could push the envelope.

My combo is 4.155 bore; 3.825 stroke, zero deck, .040 head gasket, TRW flat top pistons with 3 cc valve reliefs, and 70 cc Edelbrock aluminum heads. ZZ9 hydraulic roller that I degreed in at 114, which is two degrees retarded from straight up.

I'm running 7 degrees of initial lead and a TPIS Stage V PROM that has been tweaked slightly from TPIS's original programming. 35/45 psi fuel pressure, which I plan to increase today to 40/50 to bring up my WOT O2 mv readings.

Using Diacom and a laptop and after playing around with fuel pressure and initial lead a lot, I'm finally getting virtually NO part throttle ping or knock or knock retard and at WOT I'm getting 2 degrees of knock retard at the instant the throttle is floored. That 2 degrees is added back in after several seconds. Using pump 93 octane gas too with no octane booster additives.

Coolant temps remain under 190, and oil temps are at about 200 until I really begin to work the engine hard. The oil temps then climb to alarmingly high levels - 250 Degrees F - so an oil cooler is next on my list.

I've read 9.5 for iron heads and 10.5 for aluminum as the max too, but recently I've come across several tech articles that have raised the bar a full point or more with proper tuning.

Hope this helps.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

Sounds like a great combo. I built a 406 with 10.59 with iron heads and run 92 oct. with no deto. Zero deck and a .039" head gasket is the trick as you know.

-Auggie-